The invention relates to the field of cryogenic air separation and more particularly to cryogenic methods in which an oxygen-rich liquid bath has to be boiled.
The cryogenic distillation of air is carried out in distillation columns, and in the sump of some of these columns an oxygen-rich liquid is collected, in particular in the low-pressure column of a system of columns, such as a double air separation column. This oxygen-rich liquid is continuously boiled so as to provide reboil for the column, by means of a reboiler that is installed in the sump and fed with a heat-transfer fluid, such as the gaseous nitrogen collected at the top of the column.
This boiling of the oxygen progressively results in a progressive increase in the concentration in the liquid bath treated by the reboiler of impurities heavier than oxygen. These compounds include light hydrocarbons, CO2 and nitrogen oxides. This concentration is dangerous long term, since a threshold may then be reached above which, in certain zones of the reboiler where the liquid oxygen is completely boiled off, a deposit of hydrocarbons in the pure state may be produced on the reboiler, resulting in combustion of said hydrocarbons. This combustion may propagate to the aluminum which, for cost and energy efficiency reasons, is generally the base material from which the reboiler is made. Moreover, the build-up of inert compounds may also be dangerous when these compounds solidify in a quantity such that they block the channels of the reboiler. It is then necessary to shut down the installation in order to restore it to correct operation.
A partial solution to this problem could be to replace the aluminum reboiler with a copper reboiler, which runs no risk of catching fire in contact with hydrocarbons. However, this solution would be expensive, in particular because the exchanger would have to have substantially greater dimensions, for the same performance, than an aluminum exchanger.
Another solution, conventionally adopted, consists in purging a portion of the oxygen-rich liquid. Such a purge takes place naturally if the installation is used to produce liquid oxygen or gaseous oxygen at high pressure, by what is called the “internal compression” method, or low-pressure gaseous oxygen. However, if the oxygen is withdrawn from the column above the reboiler (something which is the case in installations producing krypton or xenon), or if the liquid oxygen withdrawn is only partially vaporized and if its unvaporized portion is sent back into the column, the problem rises in the same manner. Under these conditions, it is necessary either to purge a large stream of liquid oxygen, to send it through absorbers, in order to strip it of its impurities, and to send it back into the reboiler, or to withdraw only a small stream of liquid oxygen, but to discharge it to the outside of the system without utilizing it. Since this latter solution is costly in terms of wasted material and energy, it is beneficial to minimize as far as possible the fraction of liquid oxygen purged.
If the air treated by the cryogenic distillation installation is very clean, the purge stream may be as low as 0.01% of the total treated air stream. However, in common practice the purge stream is from 0.1 to 0.2% of the total treated air stream. The lower the purge stream, for the same initial air purity, the higher the risk of a dangerous build-up of hydrocarbons and other impurities in the oxygen-rich liquid. It is estimated in general that, with a purge stream of 10% of the total treated air stream or higher, there is no longer any danger in using an aluminum reboiler.
One solution proposed by the document WO-A-99/39143 consists in purging a fraction of oxygen-rich liquid that is sufficiently large to ensure safe operation of the reboiler and in sending the purged liquid into a second reboiler external to the installation, in which high impurity contents of the concentrated liquid found therein can be tolerated and in managing the corresponding risk. This external reboiler may be periodically warmed to a relatively high temperature so as to remove the impurities that are present therein.